Silicone pressure sensitive adhesives, hereinafter also referred to as silicone PSAs, are useful in a variety of applications by virtue of their unique properties, including excellent adhesive and cohesive strength, high tack, very low alpha particle emissions, good moisture resistance, good electrical properties, high ionic purity, and good adhesion to low energy substrates. For example, silicone PSAs are widely used in adhesive tapes, bandages, low-temperature backings, transfer films, and labels. Moreover, silicone PSAs are used in the assembly of automotive parts, toys, electronic circuits, and keyboards.
Addition-curable silicone compositions useful for preparing silicone pressure sensitive adhesives are known in the art. For example, silicone compositions containing an alkenyl-containing polydiorganosiloxane, an organopolysiloxane resin, an organohydrogenpolysiloxane, and a hydrosilylation catalyst are disclosed in U.S. Pat. No. 5,290,885 to Vincent et al.; U.S. Pat. No. 5,366,809 to Schmidt et al.; U.S. Pat. No. 3,983,298 to Hahn et al.; U.S. Pat. No. 5,399,614 to Lin et al.; U.S. Pat. No. 5,100,976 to Hamada et al.; U.S. Pat. No. 5,446,532 to Wengrovius et al.; U.S. Pat. No. 4,774,297 to Murakami et al.; U.S. Pat. No. 4,988,779 to Medford et al.; and U.S. Pat. No. 5,292,586 to Lin et al.
However, conventional silicone PSA compositions exhibit little or no thixotropy, rendering them unsuitable for some applications, such as fabrication of certain electronic packages, that require formation of adhesive deposits having critical dimensions on well-defined regions of a substrate. Under the shear conditions typically encountered during application, viscous silicone PSA compositions exhibit a pronounced tendency to adhere to the surface of the dispenser or coating device, forming a continuous string of adhesive between the device and the deposit. This condition, referred to herein as "stringing", can result in contamination of the substrate. Although, stringing is less conspicuous in the case of lower viscosity silicone PSA compositions, such compositions may sag or flow beyond the initial boundaries of the deposit. Furthermore, relatively thick films cannot be readily prepared in a single application from low viscosity silicone PSA compositions.
For the reasons stated above there is a need for a thixotropic addition-curable silicone composition that cures to form a pressure sensitive adhesive.